Having read all the threads so far the one interesting thing is the level of English written. The one post stating that it is not done by a native English speaker has a higher level of correct usage than many of the other postings. Would it have been kinder to say sorry your English is not good enough (and that comment can also apply to the person claiming to live in London).
If your written grammar is representative of your spoken language then I would not want you to have taught my kids, both of my children did go to International schools in non English speaking countries. Why should schools in Shenzhen have to accept less than schools, in say Europe, regarding the quality of English spoken (and written). Commenting on one side of the story, with no knowledge of the other, cannot been seen as wise, making judgements less so. This, of course, is only my opinion. However in my experience the truth rarely is revealed by one side alone.
Bitterness and honesty rarely go together.
As a teacher myself I have often taken over classes previously taught by non native English speakers (often they have been wonderful people with a large amount of knowledge but poor English skills) and the problems they have left, for no other reason that heavy accents or poor grammar or limited vocabulary, have taken a lot of clearing up. That said I also followed a heavily accented English guy who's students also couldn't understand him who left the same problems.
It would appear the school was not honest in its reasons for discontinuing your employment, why they did this we can only speculate.
Think on this, dismissal for visa reasons can be disclosed openly to new employers, if say it was a competency matter which could effect future employment would you have preferred them to be honest? Cruel to be kind or politics? That is a judgement I do not have the information to make